There is a demand for underwater cutting of metals, stone, and other materials for such things as mining, salvage, rescue work, infrastructure development, petroleum exploration and development, and environmental remediation. Underwater work environments are among the most difficult operating areas for cutting materials. Problems with hydrostatic pressure, high liquid viscosity (compared to air), water's high thermal and electrical conductivity, and the lack of visibility all hamper conventional cutting technologies. Non-limiting applications for safe underwater cutting systems include cutting underwater on shipwrecks and pipeline; clearing passageways through rocks for underwater communications and electrical power infrastructure; disposal of discarded military munitions (DMM), etc. Oxy-arc, oxy-fuel, oxy-hydrogen and underwater arc cutting can be used to cut steels underwater at limited depths. Mechanical drills and cutting tools, such as circular, ring, band, wire, and abrasive saws can also by used underwater with varying degrees of success. None of these methods are easy to perform underwater and all have limitations that restrict their use. They are also generally hazardous to use around explosive materials, which are all too frequently found underwater.
One conventional method of disposing of underwater munitions is to detonate them in-situ using highly skilled divers to place the necessary explosive charges. Unfortunately, fish and marine mammals such as whales, dolphins, and porpoises can be killed or seriously injured up to several kilometers from an underwater detonation due to the effects of explosive shock overpressure. Abrasive entrainment waterjets have the potential of providing a safe and environmentally friendly alternative to conventional underwater cutting technologies if certain obstacles can be overcome. Such obstacles include being able to feed a substantially steady flow of abrasive to the cutting head, can be overcome.
The word “waterjet” is an ambiguous term used to broadly describe essentially any process that expels a liquid, regardless of pressure or fluid chemistry, through an orifice to form a fluid jet. The wide-ranging term of “waterjet” is used to include everything from low-pressure dental hygiene equipment to high-pressure systems incorporating abrasives that can cut through thick hardened steel and rock. In addition, a further confusion is introduced as the use of the word “water” in the term “waterjet” does not limit the application's use to only pure water (H2O) as the fluid in the waterjet. In this context the word “water” can imply any fluid, any solution, and any solid material that will flow through an orifice under pressure or any gas that liquefies under pressure, such as ammonia, to form what should more precisely be termed a “fluid” jet, but by convention is defined in the trade as a “waterjet.”
Waterjets are fast, flexible, reasonably precise, and are relatively easy to use. They use the technology of high-pressure water being forced through a small hole, typically called the “orifice” or “jewel” which is typically about 0.007″ to 0.020″ in diameter (0.18 to 0.4 mm), to concentrate an extreme amount of energy in a small area. The restriction of the tiny orifice creates high pressure and a high-velocity jet. The inlet (process) water for a pure waterjet is typically pressurized between 20,000 psi (138 MPa) and 60,000 psi (414 MPa). This is forced through a tiny hole in the jewel,). This creates a very high-velocity, very thin jet of water traveling as close to the speed of sound.
Abrasive slurry waterjet, also known as an abrasive suspension jet, typically uses a hopper filled with abrasive, water, and a slurrying or suspension agent. This combined mixture is then pressurized and forced through the orifice of the waterjet cutting head. The abrasive slurry system must keep the abrasive in constant suspension, by chemical additives or mechanical means, in order to prevent the abrasive from dropping out of suspension in the piping which leads to plugging and disabling of the system. Likewise, the flow of pressurized abrasive and water slurry mix is highly erosive to piping, valves, and fittings used in the system. In addition, one or more large pressure vessels must typically be used to contain a sufficient amount of abrasive slurry for cutting. Consequently, an abrasive slurry system is typically limited in pressure to approximately 140 MPa and normally operates at pressures closer to 70 MPa.
An abrasive entrainment waterjet uses a high velocity fluid jet, formed by pressurized water passing through an orifice (jewel) of the cutting head resulting in a partial vacuum in a mixing chamber downstream of the orifice that aspirates and entrains abrasive particles that are introduced into said mixing chamber and into the fluid jet. Abrasive entrainment waterjet technology has several advantages over abrasive slurry waterjet technology. For example, it is more reliable; it requires less maintenance; it is being able to operate at internal system pressures up to 1,000 MPa or more; it can operate in a continuous mode rather than in a batch mode; it doesn't require expensive chemical additives; and it is able to operate with significantly lower abrasive consumption.
Waterjet technology has been used underwater for cutting metals and stone. For example, waterjets were taught as being effective in underwater mining operations. See Borkowski, P. and Borkowski, J. (2011). “Basis of High-pressure Water Jet Implementation for Poly-metallic Concretions Output from the Ocean's Bottom,” Rocznik Ochrony Środowiska Selected full texts, 13, ppg. 65-82. An abrasive slurry system is taught as being capable of operating underwater as long as the internal fluid pressure is substantially higher than the surrounding hydrostatic pressure.
While the art teaches the possibility of using waterjet technology for underwater cutting, serious problems still exist and must be overcome before such technology can be used commercially, especially in deep water.